Method for improving liquid yield in a delayed coking process

ABSTRACT

A system and method for removing oil from the surface of coke left in refinery coke drums before it is cut out is disclosed. The process does not add time to the coking cycle, nor involve the addition of major equipment. A solvent is added to the stripping steam. The solvent could be either a hydrocarbon fraction such as a coker distillate, a light gas oil or another fraction having an end point below 450° C. The solvent-enhanced stripping steam becomes a very effective oil extraction vapor when injected into the coke drum.

CROSS-REFERENCE TO RELATED APPLICATIONS

This Application claims priority to Provisional Application Ser. No. 61/111,766 filed Nov. 6, 2008, the content of which is hereby incorporated into this application by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

None.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to methods of refining liquids. More specifically, the invention relates to the field of improving liquid yield in a delayed coking process.

2. Description of the Related Art

The delayed coking process has been used in the petroleum refining industry for a considerable time. Petroleum coke was first made by the pioneer oil refineries in Northwestern Pennsylvania in the 1860's and the first delayed coker was build in 1929. Delayed coking is a thermal cracking process used in petroleum refineries to upgrade and convert petroleum residuum (bottoms from atmospheric and vacuum distillation of crude oil) into liquid and gas product streams leaving behind a solid concentrated carbon material, petroleum coke. A fired heater with horizontal tubes is used in the process to reach thermal cracking temperatures of 905 to 941° F. With short residence time in the furnace tubes, coking of the feed material is thereby “delayed” until it reaches large coking drums downstream of the heater. Coking takes place in the carbonizing mass in the lower portion of the coking drum during the delayed residence of the heated feed in the drum.

The delayed coker is the only main process in a modern petroleum refinery that is a batch-continuous process. The flow through the tube furnace is continuous with the feed stream being switched between two drums. One drum is on-line filling with coke while the other drum is being steam-stripped, cooled, decoked, pressure checked, and warmed up. Important steps of the drum cycle are as follows:

Drum Cycle Hours Steam to Fractionator 0.5 Steam to Blow Down 0.5 Depressure, Water Quench and Fill 4.5 Drain 2.0 Unhead Top and Bottom 0.5 Cutting Coke 3.0 Rehead/Steam Test/Purge 1.0 Drum Warm-Up (Vapor Heat) 4.0 Total Time 16.0

The temperature, pressure and other conditions are adjusted to maximize the yield of the desired liquid products which are formed during the reaction and which are removed by steam stripping as the reaction proceeds. At the end of the coking reaction, the coke which is left behind in the drum, is removed while the feed from the heater is switched to another drum.

One primary object of the coking process is to upgrade the residual feedstock and so to obtain relatively lighter products of greater value which may be used as feedstock for catalytic cracking units e.g. a fluid catalytic cracker (FCC). To ensure that the liquid product from the coker is of suitable quality, the product from the coker is usually fractionated and the bottoms fraction, typically boiling above 370° C. is recycled. The recycled stream generally constitutes about one quarter of the fresh feed.

Conventionally, attempts have been made to improve the yield of more highly valued, lighter products by changing the nature of the feedstock. U.S. Pat. No. 4,455,219 issued to Janssen discloses a method where the conventional delayed coking process was modified by minimizing the amount of normal heavy recycle used, and by adding a lower boiling range stream from the coker fractionator. U.S. Pat. No. 4,518,487 issued to Graf et al. discloses a process that replaces all of the heavy recycle with a lower boiling range diluent hydrocarbon fraction. U.S. Pat. No. 4,661,241 issued to Dabkowski describes an improvement in liquid yield and selectivity obtained by adding a solvent or diluent to the feedstock is which may be used in conjunction with a reactive or nonreactive gas such as nitrogen, steam, hydrogen or hydrogen sulfide. Finally, U.S. Pat. No. 5,645,712 issued to Roth discloses a process where coke drum temperatures are increased thereby increasing liquid yield by the addition of a heated non-coking hydrocarbon diluent. U.S. Pat. No. 4,404,092 issued to Audeh et al. discloses that liquid yield may be increased and coke yield decreased by controlling the temperature profile of the coking drum.

In summary, the improvements taught by prior art seek to improve liquid yield at the expense of coke yield by increasing and/or controlling the nature of the feedstock and temperature of the coke drum. Although these improvements to the coking process produce greater liquid yields at the expense of coke, there remains an unavoidable zone above the carbonizing mass where the temperature remains in equilibrium with the vaporization temperature of high molecular weight heavy gas oil following the quench cycle. Consequently, a layer of volatile material remains on the surface of the hardened coke after the quench cycle. Measurements have confirmed that the amount of volatile material may be as high as 12-15 percent by weight.

SUMMARY

Disclosed is a process for removing residual liquid oil from the surface of coke in a vessel. The process, in some embodiments, includes (i) deriving steam that is otherwise used during the “steam to fractionator” drum cycle; (ii) providing a hydrocarbon solvent; (iii) delivering said steam into a coke drum containing said liquid oil on said surface of coke; (iv) introducing a said hydrocarbon solvent into the steam being delivered; and (v) removing said liquid oil in a vaporous effluent from said coke drum while the steam and hydrocarbon solvent are being delivered.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Illustrative embodiments of the present invention are described in detail below with reference to the attached drawing FIGURE, which are incorporated by reference herein and wherein:

The FIGURE is a schematic diagram showing the injection equipment of the present invention.

DETAILED DESCRIPTION

The disclosed processes intend to reduce or eliminate the valuable liquid that remains with the coke fraction from the surface of hardened coke in the coke drum. In summary, the process is executed by (i) providing a steam source—more specifically using steam provided during the aforementioned “steam to fractionator” and “steam to blowdown” portions of the drum cycle; (ii) providing a hydrocarbon solvent source; (iii) delivering steam from said steam source to said coke drum; (iv) introducing the hydrocarbon solvent from the hydrocarbon solvent source into the steam delivered; and (v) removing vaporous effluent from said coke drum while the steam and hydrocarbon solvent are being delivered.

More specifically, the invention involves incorporating the aforementioned process into existing coke drum operations by injecting any of various solvents especially of single or multi-component hydrocarbon materials, especially in the range of C₁ to C₅₀ hydrocarbons into the high-pressure steam used during the coke drum steam quench cycle to solubilize and remove oil that has accumulated on the hardened coke surface. The described process is particularly well-suited to contacting large areas of hardened coke with relatively little hydrocarbon. Once the solvent chemistry has been administered, the coke drum is depressured, water filled and drained as part of the normal coke drum batch process. The removed oil/effluent is then further processed into valuable products.

The conventional methods improved the yield of more highly valued liquids at the expense of the lesser valued coke, but none solved the problem that residual liquid oil inevitably remained on the surface of the hardened coke. Because of this, the highly valued liquid oil is typically removed and sold at the lower price of petroleum coke. Further, none of the conventional methods are executed during the steam steps of the drum cycle.

In a standard delayed coking process, after the drum is filled with hot feedstock, steam is applied to the drum to transfer heat from the hot bottom section of the coke bed to the unconverted liquid present at the top of the coke drum. Adequate steam stripping increases the amount of recovered gas oil yield while at the same time reduces the amount of volatile matter and pitch left in the top section of the coke drum. After the steam has been flowing up through the coke bed for about thirty minutes with the vapors going to the fractionator, the vapor line is vented to the blowdown system.

The steam stripping process removes much, but not all of the unconverted liquid at the top of the coke bed. Thus, following the steam cycle, there remains an unavoidable zone above the carbonizing mass where the temperature remains in equilibrium with the vaporization temperature of high molecular weight heavy gas oil. Consequently, a layer of high molecular weight heavy gas oils—more specifically, hydrocarbons having carbon numbers predominantly in the range of C13 to C60 with a boiling range of 650-1200 F—remain on the surface of the hardened coke after the steam stripping cycle. Measurements have confirmed that the amount of these high molecular weight heavy gas oils may be as high as 12-15 percent by weight. This represents valuable matter which would conventionally be wasted.

A system for use in the disclosed processes is disclosed in the FIGURE. The cycle for the coking process involves warming up the drum (e.g., one of drums 10 and 20) using vapor heat, on-line filling, steam stripping, water cooling, draining and coke cutting. In this enhanced process, a steam source 80 is applied through valve 40 before the feedstock is switched from one drum to the other using 3-way valve 30 and immediately after the switch to avoid hot spots in the coke bed. Steam stripping also serves to transfer heat from the hot bottom section of the coke bed to the unconverted liquid present at the top of the coke drum. Adequate steam stripping increases the amount of recovered gas oil yield while at the same time reduces the amount of volatile matter and pitch left in the top section of the coke drum. Stripping steam is added to the coke drum by opening valve 40 and allowed to flow up though the coke bed 100. After the steam has been flowing through the coke bed for about thirty minutes with the vapors flowing to the fractionator through pipe 110, the vapor line is vented to a blowdown system through 3-way valve 60 and pipe 120. As the steam is applied in this manner, solvent is added to the stripping steam from pipe 90 by opening valve 50. Solvent is run with the stripping steam for about 30-45 minutes in one embodiment.

The introduction of the hydrocarbon solvent solubizes the heavy oil remaining on the surface of the coke bed 100. It does so by increasing the partial vapor pressure of the residual volatile material. This increase causes it to be removed from the coke drum with the stripping steam and sent to the fractionator through pipe 110 or to the blowdown system 120. In either case, the solubized remainder is returned to the refinery for conversion into higher value products. Thus, the coke is more completely stripped.

The present invention accomplishes the above described benefits using various solvents. This may be achieved by direct addition of the desired solvent to the stripping steam (the FIGURE). Solvents which may be used include any naturally occurring, synthetic or processed organic solvents (i.e. aliphatic, paraffinic, isoparaffinic, aromatic, naphthenic, olefinic, dienes, terpenes, polymeric or halogenated), either as single compounds or multicomponent materials. Some examples include natural terpenes and their hydrogenated derivatives or any individual hydrocarbon or hydrocarbons or even a virgin untreated hydrocarbon having requisite characteristics, but usually is a hydrocarbon fraction obtained as a product or by-product in a petroleum refining process. Alternatively, the solvents used may be obtained directly from the coker unit or derived from other sources within the refinery. As is known in the art, coker naphtha from the coking fractionator is a solvent which may be tapped from the coking process itself. Furthermore, aromatic solvents (toluene, xylene, mixed xylenes), virgin naphtha, terpenes and hexanes are solvents which might be obtained from other refining processes in the facility. Combinations of solvents as described above might be used as well.

Regardless, the end point of hydrocarbon solvents used in this way should not be more than 450° C. (about 850° F.), and generally the solvents will be from C₁ to C₅₀ hydrocarbons. Generally, the solvent will be a distillate boiling range material, i.e. having a boiling range from about 165° C. to 350° C. (about 330° F. to 650° F.), and within this range may be either a light or a heavy distillate. However, more volatile hydrocarbons may be used, for example, hydrocarbons in the gasoline boiling range or even dry gas, might be used as well.

The solvent or combination of solvents are added to the stripping steam in proximity to the coking drums and the actual point selected will depend upon the nature of the physical arrangement of the coking unit. The amount of hydrocarbon solvent added to the steam will generally be from 1 to 40 weight percent, preferably 5 to 25 weight percent.

Many different arrangements of the various components depicted, as well as components not shown, are possible without departing from the spirit and scope of the present invention. Embodiments of the present invention have been described with the intent to be illustrative rather than restrictive. Alternative embodiments will become apparent to those skilled in the art that do not depart from its scope. A skilled artisan may develop alternative means of implementing the aforementioned improvements without departing from the scope of the present invention.

It will be understood that certain features and subcombinations are of utility and may be employed without reference to other features and subcombinations and are contemplated within the scope of the claims. Not all steps listed in the FIGURE need be carried out in the specific order described. 

1. A process for removing residual liquid oil from a surface of coke in a vessel, said process comprising: (i) deriving steam from a steam source; (ii) providing a hydrocarbon solvent; (iii) delivering said steam into a coke drum containing said liquid oil on said surface of coke; (iv) introducing a said hydrocarbon solvent into the steam being delivered; and (v) removing said liquid oil in a vaporous effluent from said coke drum while the steam and hydrocarbon solvent are being delivered. 